Sound absorbing foam

ABSTRACT

Thermoplastic foams with an average cell size of greater than one and one-half millimeters and an average noise reduction coefficient in excess of 0.3 have improved sound attenuating properties when they include a Helmholtz resonator, a quarter-wave attenuator or both.

CROSS REFERENCE STATEMENT

[0001] This application claims the benefit of U.S. Provisional Application No. 60/267,533, filed Feb. 9, 2001.

BACKGROUND OF THE INVENTION

[0002] The present invention generally relates to polymeric foam materials that have acoustic absorption properties. The present invention more particularly relates to such materials that have Helmholtz resonators, quarter wave attenuators or both defined therein. The present invention still more particularly relates to such materials wherein the Helmholtz resonators and quarter wave attenuators absorb more than one frequency octave band.

[0003] United States Patent (U.S. Pat. No.) 5,892,187 discloses acoustical attenuating devices, particularly automotive headliners, that have tunable resonant chambers defined therein. The chambers, preferably Helmholtz resonators, are embedded within a support member that provides both strength and rigidity to the device. The support member is typically formed from a single sheet that has been embossed with a plurality of cavities open on a side opposite a bulk layer of filler material. The filler material may be a fibrous material such as polyester fiber or an acoustical foam. The sheet has defined therein a plurality of orifices, at least one of which communicates with each cavity. A backing sheet covers the cavities to define a plurality of resonant chambers. The backing sheet can be made of any material with sufficient structural integrity to support the support member.

[0004] U.S. Pat. No. 5,959,265 discloses a sound absorber comprising several tubular resonators, preferably of different lengths. At least one sound orifice of the resonators is adjacent to a sound-reflecting surface. The resonators or cavities should be formed from air-tight, smooth and acoustically hard materials as opposed to soft and flexible materials. Suitable acoustically hard materials include a metal or plastic sheet or a ceramic material. Quarter wave absorber blocks can be fabricated from a variety of materials ranging from rigid plastics, through open-pore or closed-pore foams, such as aluminum foam, to coated papers or foils, especially aluminum foil.

[0005] U.S. Pat. No. 6,033,756 discloses a sound attenuating element that comprises a closed cell foam layer having a plurality of channels extending therethrough. The closed cell foam layer is spaced apart from an opposing surface to define an air gap that is preferably filled with an open cell foam or a fibrous material. Each combination of a channel and an associated portion of the air gap effectively forms a Helmholtz resonator even if there is no dividing element such as a honeycomb of a plastic or a resinated paper disposed within the air gap. The closed cell layer may be fabricated from a variety of materials including polyurethane, polyethylene, polypropylene, neoprene, rubber or phenolic foams. Preferred closed cell layers have sufficient rigidity to be self-supporting. Channel spacing affects where absorption peaks occur. A 20 millimeter by 20 millimeter square array of seven three millimeter channels provides an absorption peak at approximately 500 Hertz (Hz) whereas a 10 millimeter by 10 millimeter square array of 16 three millimeter channels provides an absorption peak between 800 Hz and 1000 Hz.

[0006] EP 454,949 discloses an air conducted sound absorbing element that has a plurality of Helmholtz resonators with different resonant frequencies arranged over the full area of its surface directed towards a sound source. The surface of the element, designed as a panel absorber, encloses the resonators with a form fit while leaving their openings free. The material of the resonators and the material of the panel absorber are identical.

[0007] EP 729,532 discloses acoustic insulating panels or elements, especially multilayered panels that have a soft core with hollow profiles. The multilayered panel has two outer facings and a soft synthetic core material that is a continuous foam core material having cavities. The core material has intimate contact with both outer facings in alternate patterns thereby providing gaps between the core material and the opposing outer facing. Certain of the hollow profiles can function as Helmholtz resonators, particularly by giving suitable dimensions to neck 5 (shown in FIG. 2).

[0008] WO 00/34595 discloses acoustical panels for sound control. Such a panel includes a sandwich or composite of a first layer of molding material, a honeycomb of cells and a second layer of molding material. The first layer has one or more apertures in it. These apertures convert the corresponding cells into Helmholtz resonators. The Helmholtz resonators may be tuned to the same, or different, frequencies. Examples of molding media for the first and second layers include medium density moldable fiberglass, open-cell melamine foam or mineral wool board. Aperture formation may occur by rolling an intermediate panel formed of the first and second layers and the honeycomb across a perforating drum having piercing needles arranged at a desired pitch or spacing.

[0009] While certain polymeric foam bodies exhibit sound attenuation properties, further improvement in sound attenuating capacity of such foam bodies would be desirable. An improvement in sound attenuation capacity over a frequency range of 250 to 2000 Hz would be particularly desirable. An improvement in sound attenuation capacity over a number of frequencies rather than a single characteristic frequency would be even more desirable.

SUMMARY OF THE INVENTION

[0010] One aspect of the present invention is an improved, sound-attenuating cellular thermoplastic polymer foam body, the foam body having

[0011] a. an average cell diameter of at least one and one-half millimeters and

[0012] b. an impedance tube average noise reduction coefficient of at least 0.3, wherein the improvement comprises at least one Helmholtz resonator, each Helmholtz resonator being disposed within the foam body, having a neck in fluid communication with an external surface of the foam body and being spaced apart from each other, whereby each Helmholtz resonator improves sound absorption coefficients about a Helmholtz center peak frequency band, such that at least one adjacent octave band has a sound absorption coefficient within 0.4 units of a peak sound absorption coefficient and increases the impedance tube average noise reduction coefficient by at least 0.1 relative to the impedance tube average noise reduction coefficient of the same foam body without any Helmholtz resonators.

[0013] In a related aspect, the foam body also includes at least one quarter wave attenuator, the quarter wave attenuator(s) being spaced apart from each other and having a diameter that exceeds the average cell diameter, each attenuator beginning at a surface of the foam body and extending into, but not through, the foam body, whereby the quarter wave attenuator(s) increase(s) the impedance tube average noise reduction coefficient by at least 0.1 relative to the impedance tube average noise reduction coefficient of the same foam body without any quarter wave attenuator(s), and each quarter wave attenuator improves sound absorption coefficients about a quarter wave attenuator center peak frequency band, such that at least one adjacent octave band has a sound absorption coefficient within 0.4 units of a peak sound absorption coefficient.

[0014] A second aspect of the present invention is an improved, sound-attenuating cellular thermoplastic polymer foam body, the foam body having

[0015] a. an average cell diameter of at least one and one-half millimeters and

[0016] b. an impedance tube average noise reduction coefficient of at least 0.3, wherein the improvement comprises at least one quarter wave attenuator, the attenuator(s) being spaced apart from each other and having a diameter that exceeds the average cell diameter, each attenuator beginning at a surface of the foam body and extending into, but not through, the foam body, whereby the quarter wave attenuator(s) increase(s) the impedance tube average noise reduction coefficient by at least 0.1 relative to the impedance tube average noise reduction coefficient of the same foam body without any quarter wave attenuator(s), and each quarter wave attenuator improves sound absorption coefficients about a quarter wave attenuator center peak frequency band, such that at least one adjacent octave band has a sound absorption coefficient within 0.4 units of a peak sound absorption coefficient.

[0017] In an aspect related to both the first and second aspects, the foam body further includes one or more surface modifications selected from wedge-shaped indentations, polyhedral indentations, and incisions that penetrate the surface of the body, but do not extend through the body.

Brief Description of the Drawings

[0018] FIG. (FIG.) 1 is a schematic cross-sectional view of a Helmholtz resonator.

[0019]FIG. 2 is a graphic portrayal of impedance tube testing data presented in Table 1. This shows baseline (no Helmholtz resonator or quarter wave attenuator) data for a polyolefin foam, as well as data for the same polyolefin foam with Helmholtz resonators and the same foam with quarter wave attenuators.

[0020]FIG. 3 is a graphic portrayal of impedance tube testing data presented in Comp Ex A. This shows baseline (no Helmholtz resonator or quarter wave attenuator) data for extruded polystyrene foam, as well as data for the same polystyrene foam with Helmholtz resonators and the same polystyrene foam with quarter wave attenuators.

[0021]FIG. 4 is a graphic portrayal of the impedance tube testing data presented in FIGS. 2 and 3.

Description of Preferred Embodiments

[0022] The theory and physics of Helmholtz resonators are known. Thus, only a brief discussion of the theory and physics will be provided in regard to FIG. 1. [Copy FIG. 12 from WO 00/34595.] A cross-sectional view of a basic Helmholtz resonator 1200 is illustrated in FIG. 1. The volume, V, of air in chamber 1202 of Helmholtz resonator 1200 is linked to environment 1212 (containing a sound source) outside resonator 1200 via an aperture 1206. Aperture 1206 has a cross-sectional area, S, and a length, L, indicated via items 1208 and 1210, respectively, in FIG. 1.

[0023] When sound impinges on aperture 1206, air in the neck of the aperture will be induced to vibrate. In turn, this causes the volume of air in the cavity to undergo periodic compression and expansion. The friction between the air particles in aperture 1206, and the resistance to air flow associated with the neck itself, cause the energy in sound waves to be absorbed. The efficiency of this absorption is at a maximum when resonance occurs, with the efficiency diminishing at frequencies above and below the resonant frequency.

[0024] The general equation governing performance of a Helmholtz resonator is:

[0025] F=(C/2Π)S^(½)(LV)−^(½)

[0026] where F=resonant frequency (Hz)

[0027] C=velocity of sound (meters per second (m/sec))

[0028] L=depth of hole (m)

[0029] S=cross-sectional area of hole (m²) V=volume of chamber (m³)

[0030] By appropriately selecting V, L and S, the resonant frequency of the Helmholtz resonator can be controlled.

[0031] Another description of a Helmholtz resonator (sometimes called a cavity resonator) is an enclosed volume of air that is connected to a short, open neck. When sound waves strike the neck of the resonator, they set the air within it in motion; in the low-frequency range, the air in the neck behaves somewhat like a solid plug that moves back and forth, compressing the air in the enclosed volume of the resonator. As the plug moves back and forth, acoustic energy is converted to heat as a result of friction of the movement of air along the walls of the neck. Maximum sound absorption occurs at the resonance frequency of the resonator which is determined by a combination of: (a) mass of air in the neck and (b) a “spring” furnished by the air that is compressed in the enclosed volume. The resonance frequency ƒ of a conventional Helmholtz resonator is given by ƒ=2160S/(vV)^(½), where V is the enclosed volume of the resonator, S is the cross-sectional area of the neck, and v is the volume of the neck, and where all dimensions are expressed in inches. The corresponding equation in metric units is given by ƒ=5500S/(vV)^(½) where all dimensions are expressed in centimeters.

[0032] A quarter wave attenuator (QWA) is preferably a circular hole, cavity or chamber that is in fluid communication with an external surface of the foam body, with a depth not to exceed the depth of the material. While the QWA may have another shape, such as square or rectangular, such shapes may lead to a build up of standing waves within the QWA, thereby reducing its effectiveness. A QWA functions through passive wave cancellation. At a depth of one quarter wavelength, a reflected wave in the cavity cancels an incoming wave thereby effectively resulting in a cancellation of acoustic energy. A QWA typically has a very narrow performance band around a center peak absorption frequency. A calculation of cavity depth is as follows.

[0033] D=cf*4, where

[0034] c=speed of sound

[0035] D=depth of cavity

[0036] f=desired frequency absorbed (Hz)

[0037] Thermoplastic resins suitable for purposes of the present invention include all types of thermoplastic polymers and blends that are foamable by extrusion processes. Examples of thermoplastic polymer resins suitable for the present invention include, but are not limited to, polyolefin resins, including polyethylene resins, polypropylene resins, as well as blends of ethylene-styrene interpolymer (ESI) resins with polyolefin resins, such as blends of polyethylene and ESI or polypropylene and ESI, polyethylene resins, copolymers of polyethylene resins, and blends of polyethylene resins being preferred. Examples of such resins are low density polyethylene resins.

[0038] Thermoplastic foams and crosslinked thermoplastic foams may be prepared by techniques and procedures well known to one of ordinary skill in the art and include extrusion processes as well as batch processes using a decomposable blowing agent and cross-linking, with extrusion processes being preferred. By way of example, WO 00/15697 describes some of such techniques and processes at page 8, line 20 through page 12, line 32. The teachings of WO 00/15697 are incorporated herein to the extent allowed by law.

[0039] Foams suitable for purposes of the present invention may be characterized by two primary features, average cell diameter and impedance tube noise reduction coefficient. The foams may also be characterized by their density. Foam densities desirably range from 10 kg/m³ to 300 kg/m³, with foams having densities of from 15 kg/m³ to 100 kg/m³ being preferred, and foams having densities of from 15 kg/m³ to 60 kg/m³ being particularly preferred. The foams have an average cell size of at least 1.5 millimeters (mm), desirably at least 2 mm, preferably at least 4 mm and more preferably at least 6 mm. The average cell size desirably does not exceed 15 mm, with an average cell size of no more than 12 mm being preferred and no more than 10 mm being more preferred. Foam impedance tube NRCs are desirably at least 0.3, preferably at least 0.5, more preferably at least 0.7. Such NRC's are desirably less than 0.9. In addition, the foams may be open or closed celled. If desired, closed cell foams may be subjected to means sufficient to open at least a portion of closed cells contained in the foam. Unless otherwise stated herein, all ranges include both endpoints that establish the range.

[0040] Closed cells may be opened or converted to open cells by a variety of techniques including, but not limited to, perforation, slicing, compression, or combinations thereof. Typically, perforation comprises puncturing the base foam with one or more pointed, sharp objects. Suitable pointed, sharp objects include needles, spikes, pins, or nails. In addition, perforation may comprise drilling, laser cutting, high pressure fluid cutting, air guns, or projectiles.

[0041] Adding a Helmholtz resonator (HR) to a thermoplastic foam body may be by any means sufficient to define a cavity or chamber in the foam body of desired dimensions, but will typically include routing, drilling, machining, hot wire cutting, slicing, or combinations thereof. The HR preferably produces a peak absorption frequency within a frequency range of 250 to 2000 Hz. Specific HR dimensions vary depending upon the peak absorption frequency of interest. Skilled artisans can readily determine such dimensions using the formula presented herein once they select a desired peak absorption frequency. A preferred means includes use of a conventional router with router blade selection appropriate to fabricate a cavity of desired dimensions. The chamber is connected to air outside the foam body by a neck or passageway that has a maximum cross-sectional diameter. The neck maximum cross-sectional diameter is less than the chamber's length or width where the neck intersects the chamber. The chamber and neck comprise walls of one or more cells of the foam body.

[0042] Adding a QWA to the thermoplastic foam may be by any of the means listed as suitable for defining a HR, but will typically involve simple drilling with a drill bit of appropriate diameter. Like the HR, a QWA produces a peak absorption frequency within a frequency range of 250 to 2000 Hz. Specific QWA dimensions vary depending upon the peak absorption frequency of interest. Skilled artisans can readily determine such dimensions using the formula presented herein once they select a desired peak absorption frequency.

[0043] In addition to HRs, QWAs or both, foam bodies of the present invention may further comprise one or more surface modifications selected from wedge-shaped indentations, polyhedral indentations, and incisions that penetrate the surface of the body, but do not extend through the body. Preferably, none of these surface modifications intersect, or have fluid communication with, any HR or QWA. By way of example only, a suitable wedge shape mentioned has a wedge tip angle of 36 degrees and a depth of 25 mm. Similarly, suitable incisions in the foam body are made parallel to each other at a 45 degree angle to the surface, with a depth of 25 mm measured orthogonal to the surface. The shape and depth of the surface modifications preferably vary depending upon needed acoustic dispersion. The surface modifications may be made using any of the objects described for use in perforating a foam body. Other objects or implements that may be used to make surface modifications include cutting blades, or friction wire cutters.

[0044] The following examples illustrate, but do not limit, the scope of the present invention.

Example (Ex) 1 and Comparative Example (Comp Ex) A

[0045] Use a router equipped with a bit in the shape of a Helmholtz resonator to cut HRs in a foam body. Space the HRs a distance of 43 mm apart from each other, measuring from center of neck opening to center of neck opening. The HRs have a neck with a width of 6.5 mm and a depth of 12 mm and a cavity body in fluid communication with the neck that has a width of 19 mm and a depth of 14.5 mm.

[0046] Use a drill press equipped with a 12.5 mm bit to cut holes having a depth of 33 mm in a foam body for use as QWA. Space the holes a distance of 25 mm apart from each other, measuring from hole center to hole center.

[0047] Subject foam body samples to acoustic absorption testing using a Model 4206 acoustical impedance tube and Model 3555 signal analyzer, both available from Bruel and Kjaer A/S, Naerum, Denmark. Testing measures sound absorption coefficients of absorptive materials at normal incidence over a frequency range of 50 Hz to 6.4 kilohertz using a large (low frequency) tube and a small (high frequency) tube in accordance with ASTM test method E1050.

[0048] Prepare profiled foam bodies from several different thermoplastic polymer foam bodies by cutting HRs, QWAs or both into the foam bodies using the procedures described above. Each foam body, prior to profiling, has a thickness of two inches (5.1 centimeters (cm). The thermoplastic polymer used to make the polyolefin foam of Ex 1 is a blend of 70 weight percent (wt %) low density polyethylene (density of 0.923 grams per cubic centimeter (g/cc), melt index (I²) of 1.8 decigrams per minute (dg/min)) and 30 wt % ethylene/styrene interpolymer ( DS201, a 1 dg/min melt index, 69 wt % styrene/31 wt % percentages being based upon blend weight. The polyolefin foam has an average cell size of 5.6 mm, a density of 2.0 pounds per cubic foot (pcf) (32 kilogram per cubic meter (kg/m3)) and an average impedance tube noise reduction coefficient (NRC) of 0.50. The polyolefin foam contains a plurality of perforation channels imparted by a 2 mm needle to provide a perforation channel density of 1 hole/cm2 (hole spacing of approximately 10 mm). The thermoplastic polymer for Comp Ex A is general purpose polystyrene (melt flow rate of 11 dg/min and density of 1.04 g/cc). An extruded polystyrene foam prepared from the general purpose polystyrene has an average cell size of 0.25 mm, a density of 1.8 g/cc and an impedance tube NRC of 0.02.

[0049] Subject the profiled foam bodies to sound absorption testing by measuring normal incidence sound absorption coefficient of a foam at several frequencies according to a method described in American Society for Testing and Materials (ASTM) Test E-1050. Calculate an average NRC by averaging sound absorption coefficients at four frequencies: 250 Hz, 500 Hz, 1000 Hz and 2000 Hz. Table I below summarizes average NRC data for Ex 1 and Comp Ex A before and after profiling with Helmholtz Resonators (HRs) or QWAs. Table II below summarizes impedance tube testing data for Ex 1. Table III below summarizes impedance tube testing data for Comp Ex A. TABLE I Average NRC Average without NRC with Average NRC Ex/Comp Ex Profiling HR with QWA Ex 1 0.50 0.69 0.62 Comp. Ex. A 0.02 0.21 0.05

[0050] TABLE II f (Hz) Ex 1 Baseline Ex 1 with HR Ex 1 with QWA  125 0.09 0.03 0.03  160 0.12 0.05 0.06  200 0.20 0.07 0.07  250 0.35 0.11 0.09  315 0.59 0.18 0.15  400 0.63 0.39 0.29  500 0.44 0.72 0.59  630 0.37 0.65 0.78  800 0.40 0.69 0.73 1000 0.46 0.95 0.83 1250 0.52 0.96 0.89 1600 0.70 0.98 0.96 2000 0.74 0.97 0.98 2500 0.59 0.81 0.79 3150 0.73 0.64 0.82 4000 0.61 0.56 0.90 5000 0.70 0.65 0.98 6300 0.77 0.87 0.81 Average 0.50 0.69 0.62 NRC

[0051] TABLE III Comp Ex A Comp Ex A with Comp Ex A with f (Hz) Baseline HR QWA  125 0.01 0.00 0.01  160 0.01 0.01 0.01  200 0.01 0.02 0.02  250 0.02 0.02 0.02  315 0.00 0.02 0.02  400 0.02 0.02 0.02  500 0.02 0.03 0.02  630 0.02 0.03 0.03  800 0.02 0.05 0.04 1000 0.02 0.08 0.04 1250 0.02 0.14 0.05 1600 0.03 0.37 0.06 2000 0.03 0.71 0.12 2500 0.03 0.56 0.28 3150 0.02 0.25 0.14 4000 0.04 0.19 0.08 5000 0.06 0.11 0.09 6300 0.08 0.19 0.15 Average 0.02 0.21 0.05 NRC

[0052] The data in Table I show that incorporation of HRs and QWAs into foam bodies of the present invention provides a surprising improvement in sound attenuation over a broad frequency range spanning multiple octave frequency bands. This contrasts with a single characteristic resonance frequency predicted by the general equation governing performance of a Helmholtz resonator. Comp Ex A, a small-celled, extruded polystyrene foam exhibits a typical single characteristic resonance frequency.

[0053] Results comparable to those obtained with Ex 1 are expected using other thermoplastic polymer foam bodies that have a baseline average NRC greater than 0.3 and an average cell size greater than 1.5 mm. A graphic portrayal (FIG. 2) of impedance tube testing data in Table II shows that addition of either an HR or a QWA to such a foam body produces absorption over more than one octave frequency band. A graphic portrayal (FIG. 3) of impedance tube testing data in Table III shows that HR addition to the foam body of Comp Ex A produces absorption over a single octave frequency band. 

What is claimed is:
 1. An improved, sound-attenuating cellular thermoplastic polymer foam body, the modified foam body having a. an average cell diameter of at least one and one-half millimeters; and b. an impedance tube noise reduction coefficient of at least 0.3, wherein the improvement comprises at least one Helmholtz resonator, each Helmholtz resonator being disposed within the foam body, having a neck in fluid communication with an external surface of the foam body and being spaced apart from each other, whereby each Helmholtz resonator improves sound absorption coefficients about a Helmholtz center peak frequency band, such that at least one adjacent octave band has a sound absorption coefficient within 0.4 units of a peak sound absorption coefficient and increases the impedance tube noise reduction coefficient by at least 0.1 relative to the impedance tube average noise reduction coefficient of the same foam body without any Helmholtz resonators.
 2. An improved, sound-attenuating cellular thermoplastic polymer foam body, the foam body having a. an average cell diameter of at least one and one-half millimeters and b. an impedance tube average noise reduction coefficient of at least 0.3 wherein the improvement comprises at least one quarter wave attenuator, the attenuator(s) being spaced apart from each other and having a diameter that exceeds the average cell diameter, each attenuator beginning at a surface of the foam body and extending into, but not through, the foam body, whereby the quarter wave attenuator(s) increase(s) the impedance tube average noise reduction coefficient by at least 0.1 relative to the impedance tube average noise reduction coefficient of the same foam body without any quarter wave attenuator(s), and each quarter wave attenuator improves sound absorption coefficients about a quarter wave attenuator center peak frequency band, such that at least one adjacent octave band has a sound absorption coefficient within 0.4 units of a peak sound absorption coefficient.
 3. The foam body of claim 1, wherein the foam body also comprises at least one quarter wave attenuator, the attenuator(s) being spaced apart from each other and having a diameter that exceeds the average cell diameter, each attenuator beginning at a surface of the foam body and extending into, but not through, the foam body, whereby the quarter wave attenuator(s) increase(s) the impedance tube average noise reduction coefficient by at least 0.1 relative to the impedance tube average noise reduction coefficient of the same foam body without any quarter wave attenuator(s), and each quarter wave attenuator improves sound absorption coefficients a quarter wave attenuator center peak frequency band, such that at least one adjacent octave band has a sound absorption coefficient within 0.4 units of a peak sound absorption coefficient.
 4. The foam body of claim 1, claim 2, or claim 3, wherein the foam body further comprises one or more surface modifications selected from wedge-shaped indentations, polyhedral indentations, and incisions that penetrate the surface of the body, but do not extend through the body.
 5. The foam body of claim 1, claim 2, or claim 3, wherein the average cell diameter is greater than or equal to four millimeters.
 6. The foam body of claim 4, wherein the average cell diameter is greater than or equal to four millimeters.
 7. The foam body of claim 1, claim 2, or claim 3, wherein the average cell diameter is greater than or equal to six millimeters.
 8. The foam body of claim 4, wherein the average cell diameter is greater than or equal to six millimeters.
 9. The foam body of claim 1, claim 2, or claim 3, wherein the thermoplastic polymer is a low density polyethylene resin, a polypropylene resin or a blend of a low density polyethylene resin and a polypropylene resin, or a combination of any of the above with an ethylene styrene interpolymer.
 10. The foam body of claim 4, wherein the thermoplastic polymer is a low density polyethylene resin, a polypropylene resin or a blend or a low density polyethylene resin and a polypropylene resin, or a combination of any of the above with an ethylene styrene interpolymer. 